The present invention relates generally to polyolefin foams and, more particularly, to extruded polyethylene foam sheets having a coating disposed thereon for bonding the polyethylene foam sheet to a polypropylene foam sheet.
Polyolefin foams, particularly polyethylene foams, and methods for manufacturing such foams are well known in the art. See, e.g., U.S. Pat. Nos. 5,348,984 (Lee), 5,462,974 (Lee), and 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyethylenes used is low density polyethylene (LDPE).
While polyethylene (PE) possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of PE is that extruded foam sheets made therefrom have a flexural modulus that is lower than would otherwise be desired for certain applications. For example, PE foams are commonly used to manufacture a type of flotation article known as a xe2x80x9cbodyboard,xe2x80x9d which is a generally planar foamed structure of various shapes and sizes that allows individuals to be carried forward by oceanic waves as the waves break near the shore, i.e., to ride or xe2x80x9csurfxe2x80x9d the waves. This is commonly accomplished by grasping the edges of the bodyboard with a portion of the upper body positioned above, and often in contact with, the upper surface of the board. Individuals engaging in such activity generally prefer that the bodyboard be as stiff as possible as this allows the individual to maneuver the board to control somewhat the individual""s direction of travel relative to the wave surface upon which the individual is xe2x80x9csurfing.xe2x80x9d Too much flexure in the waveboard detracts from this ability to maneuver the board along the wave surface. Accordingly, efforts have been made to increase the stiffness of the foam used to make bodyboards.
One possible solution would be to construct the bodyboard from polypropylene (PP) foam, which is much stiffer than PE foam. However, PP foam has generally been found to be overly stiff and brittle, resulting in a bodyboard that is too easily damaged (e.g., gouged) and difficult to firmly grasp and handle, particularly when wet. In addition, PP foam has a surface texture that is uncomfortable against the skin, especially when rubbed against the skin as generally occurs during bodyboarding.
Another possibility would be to form a composite structure that includes a PE foam sheet bonded to a PP foam sheet. The PE foam sheet would provide comfort, grippability, and durability, while the PP foam sheet would provide increased stiffness and performance. Unfortunately, PP and PE foams are not chemically compatible and will not readily adhere to one another. Delamination is therefore a significant problem for PE foam/PP foam laminates.
It may be possible, in certain cases, to use special chemical adhesives (i.e., glues) to bond PP and PE foams. However, such adhesives are expensive and the process of coating and bonding is tedious and not cost-effective. The process involves slow production steps and the adhesives contain volatile organic solvents that are not environmentally friendly for commercial production. Extra equipment is generally needed to recover the solvents, thus further adding to the expense and complexity of the adhesive coating process.
Accordingly, a need exists in the art for an economical and practical means for bonding PP foam to PE foam with sufficient strength that the resultant composite structure is suitable for commercial use, such as for bodyboards or other watersport applications.
That need is met by the present invention, which provides a composite structure, comprising:
a. foam sheet comprising polyolefin; and
b. a coating disposed on at least one surface of the polyolefin foam sheet, the coating comprising at least one member selected from the group consisting of ethylene/propylene rubber, homogeneous ethylene/alpha-olefin copolymer, and ethylene/acrylic acid copolymer.
Advantageously, the coating is capable of bonding the polyolefin foam sheet to a second foam sheet having a different chemical composition than the polyolefin foam sheet at a bond strength of at least about 4 lbf/inch.
In a preferred alternative embodiment of the invention, a multilayer composite structure comprises:
a. a first foam layer comprising polyethylene homopolymer or copolymer;
b. a second foam layer comprising polypropylene homopolymer or copolymer; and
c. a coating disposed between and bonding the first and second foam layers together at a bond strength of at least about 4 lbf/inch, the coating comprising at least one member selected from the group consisting of ethylene/propylene rubber, homogeneous ethylene/alpha-olefin copolymer, and ethylene/acrylic acid copolymer.
In accordance with yet another embodiment of the invention, a method for making a composite structure is provided, comprising:
a. providing a foam sheet comprising polyolefin; and
b. coating at least one surface of the polyolefin foam sheet, the coating comprising at least one member selected from the group consisting of ethylene/propylene rubber, homogeneous ethylene/alpha-olefin copolymer, and ethylene/acrylic acid copolymer,
whereby, the coating is capable of bonding the polyolefin foam sheet to a second foam sheet having a different chemical composition than the polyolefin foam sheet at a bond strength of at least about 4 lbf/ inch.
As is explained in further detail below, the invention provides a means for bonding PE foam to PP foam with a solventless coating process, i.e., via a coating that contains substantially no volatile organic solvent. Further, the resultant bond strength between the PE and PP foam sheets of at least 4 lbf/inch is sufficiently high for commercial watersport applications.